3.6	Recommendation I.335

ISDN ROUTING PRINCIPLES

It should be noted that the user specifies only the service required. The network allocates the resources to set up a connection of the specific

type as necessary to support the requested service. For certain services, additional network functions, e.g. additional lower layer function and/or higher layer functions, may be required as depicted in Figure 1/I.335. For examples of such cases refer to Recommendation I.310.

Figure 2/I.335 shows the general relationship between telecommunication services and ISDN connection types. It also shows in general the association with the actual realization of a service provision (call) by the establishment of a connection through the selection of a route.

- an appropriate association between the selected functional grouping in terms of a physical realization, i.e. the network allocates the set of connection elements necessary to realize the appropriate connection type.

The concept of connection type describes network capabilities using the attribute technique. One of these attributes is known as "Information (Transfer Susceptance". Some other attributes (e.g. "Connection Control Protocol") describe the signalling capabilities.

-	CCITT No. 6 signalling

Figure 3/I.335 shows the relationship between recommendations relating to routing

m)	Time of day

Because of varying traffic distributions during a 24 hours period, it maybe advantageous to change the call routing arrangements dependant of time of day.

4.	Generic performance parameters

Nine generic primary performance parameters are listed below. These have been developed as a result of the matrix approach described in Annex A. These parameters may be used in developing specific QOS and NP parameters:

-	Access speed

-	Access accuracy

-	Access dependability

-	Information transfer speed

-	Information transfer accuracy

-	Information transfer dependability

-	Disengagement speed

-	Disengagement accuracy

-	Disengagement dependability

Section 3.4.2 defines derived performance parameters in addition to primary parameters. Derived performance parameters are determined utilizing a function of the primary performance parameter values. Recommendation G.821 defines one such function, which identifies transitions between available and unavailable states based on a threshold for severely errored seconds. The generic derived performance parameter associated with such a function is availability.

Examples of specific primary and derived performance parameters for bearer service QOS and those for circuit-switched and packet-switched NP are provided in Annex B.